Crikey! Tooth analysis shows little to fear from crocs’ vegetarian ancestors

Vegetarian crocodiles once roamed the world: a lesson in not stereotyping on the basis of modern examples.

An artist's impression of Simosuchus clarki tucking in to its greens.

Compared with mammals, living members of the crocodile clan have exceptionally boring dentition.

From the slender-snouted gharials of India and the nocturnal caimans of South America to the saltwater behemoths of the South Pacific, crocodile teeth vary little in morphology.

All are conical and pointed. Each tooth in an animal’s mouth is almost identical to its neighbours — as befits­ a group that feeds on a mixture of fish and the occasional careless beast that strays too close to the shore, or even into the water itself.

This predilection for pointed fangs is not, however, how it has always­ been. During the days of the dinosaurs, the Jurassic and Cretaceous periods, crocodile clan members showed extraordinary dental diversity.

Many of their teeth have proved so bizarre that some palaeontologists have theorised that, far from being carnivorous, these ancien­t species might have been eating plants. A study published this week in Current Biology, by Keegan Melstrom and Randall Irmis at the University of Utah, confirms this. It also suggests that herbivory evolved in the crocodile clan on several occasions.

When trying to work out what ancient animals ate, palaeontologists usually look to modern analogs. If teeth from an extinct beast match those of a modern species, the two are quite likely to have had similar diets.

With extinct crocodilians, however, this tactic has routinely been stymied becaus­e their teeth, which are adorned with many rows of cusps and wrinkled enamel, look nothing like what is found in the mouths of animals alive today.

This has left the topic of what ancient crocodilians ate very much up for grabs. Some palaeontologists say certain species, such as Simosuchus clarki, were ­vegetarian.

To solve the puzzle, Mr Melstrom and Dr Irmis turned to Orientation Patch Count Rotated analysis. This technique scans a tooth and measures the complex­ity of its surfaces. Use of OPCR has demonstrated, in a quantifiable manner, that diet is closely related to tooth complexity. Carnivores tend to have simple teeth. Omnivores have more complex teeth. Herbivores have the most complex teeth of all.

Until now, however, the technique has been used mostly on the molars of living mammals. Mr Melstrom and Dr Irmis knew of no studies that had tested it on crocodiles. This lack of testing made sense, because living crocodilians have no complex tooth morphologies to analyse. But the strange teeth of ancient crocodiles, they reasoned, might give OPCR something to work with.

They threw 146 teeth from 16 extinct crocodilians at OPCR. For comparison, they added teeth from a modern caiman. The analysis revealed that two of the extinct species were, like the caiman, carnivorous­. But even these were notably different from modern animal­s in that one had serrated teeth and the other had triangular teeth that made contact with one another when the animal closed its mouth (something not seen in modern crocodilians).

The system identified two of the species as “durophagus”, whose teeth looked as if they would be good at crushing the shells of crabs and other armoured invertebrates. One species was identified as omnivorous. Eight, including Simosuchus, were identified by OPCR as obligate herbivores. (The other three were hard to classify, but may have been insectivo­res.)

What surprised Mr Melstrom and Dr Irmis, though, was the way herbivory mapped on to the crocodile family tree. Rather than evolving once at some point long ago and then appearing in all later species­ on that branch, it came into existence at least three times during­ the history of these reptiles.

Herbivorous crocodiles of the Jurassic and Cretaceous, then, were capable of competing with their dinosaur counterparts in a way that a modern herbivorous crocodile presumably could not with the plethora of herbivorous mammals that now exists.